1. Field of the Invention
The present invention relates to a data processor for data compression of image data.
2. Description of the Prior Art
An international standard for color still image coding by a Joint Photographic Experts Group (JPEG) is a known coding technique for image data. In the color image coding, image data of red, green and blue are converted to a luminance data V and color difference data W.sub.R and W.sub.B. The luminance data V and the color difference data W.sub.R and W.sub.B are coded separately. Then, the input data (luminance data V or color difference data W.sub.R, W.sub.B) is divided into processing blocks each of a prescribed size of pixel matrix (normally 8*8 pixel matrix), and image data in a processing block is subjected to discrete cosine transform (DCT) to generate DCT coefficients. Then, they are compressed as Huffman codes. The coded data are decoded when the document image is reproduced.
An image data of a document read by an image reader is an object of the above-mentioned coding. When an image data of a document of a single color or an achromatic color is read by an image reader, the image data will be all the same ideally. For example, in a case of a document of only red color, chroma has a certain value, but the AC components of the DCT coefficients will all be zero. On the other hand, in a case of an achromatic color image of black and white, chroma will be zero. However, errors occur actually on reading an image data because a read condition such as a quantity of exposure light or a reflectivity of a document paper may change.
As explained above, image data are usually divided into processing units of say 8*8 pixel matrix and they are processed for coding for each processing unit with discrete cosine transformation. Values of DC component and AC components of the DCT coefficients obtained after the discrete cosine transformation are affected by the existence of incorrect data caused by read errors. Therefore, if the incorrect values are decoded to reproduce a document image, a so-called block distortion occurs wherein colorfulness changes for each processing block and this deteriorates a quality of the entire reproduced document image.